Make it a little bit easier to install this package

Author: freekmurze

README.md

@@ -7,6 +7,13 @@ This repository holds all interfaces/classes/traits related to
 Note that this is not a logger of its own. It is merely an interface that
 describes a logger. See the specification for more details.
 
+Installation
+------------
+
+```bash
+composer require psr/log
+```
+
 Usage
 -----